(1) Field of the Invention
The present invention relates to a screw compressor in which surfaces of rotors are processed.
(2) Description of the Related Art
In a screw compressor, a pair of a male rotor and a female rotor is rotated while being meshed with each other in a casing, and fluid in spaces formed of the casing and the both rotors is compressed while the spaces are allowed to move in the axis direction to be decreased.
There are an oil-cooling screw compressor in which oil as fluid is supplied into a casing and an oil-free screw compressor in which no oil is supplied into a casing.
In the oil-cooling screw compressor, a male rotor and a female rotor are rotated while being brought into contact with each other through oil films. The oil-cooling screw compressor can prevent seizure of the rotors by cooling friction heat generated by rotation of the rotors using the oil.
The oil-cooling screw compressor is not suitable for use in fields such as the food industry and the semiconductor-related industry where clean air is required because oil mist is mixed with compressed air.
On the other hand, oil is not used at all in the oil-free screw compressor, and thus clean air can be supplied. However, both rotors are rotated in a non-contact state so as not to cause seizure of the rotors due to no seals of oil. Therefore, synchronous gears are attached to shaft ends of the rotors to apply rotational force to the rotors in the oil-free screw compressor. Thus, the structure of the oil-free screw compressor is complicated as compared to that of the oil-cooling screw compressor.
Further, the rotors are rotated in a non-contact state in the oil-free screw compressor. Thus, compressed air flows back to the suction side from gaps between both rotors or between the rotors and a rotor casing to possibly cause adverse effects on the performance of the screw compressor. Therefore, it is necessary for the oil-free screw compressor to minimize the sizes of the gaps between both rotors or between the rotors and the rotor casing in a non-contact state in order to improve performances such as volumetric efficiency. In fact, it is impossible to completely realize a non-contact state due to thermal expansion, mechanical processing errors, and the like. Thus, it is essential to provide a solid lubricating function for the rotor surfaces.
Therefore, coatings are generally applied on the rotor surfaces of the oil-free screw compressor. By providing the coatings on the rotor surfaces, scuffing or seizure can be prevented, and the sizes of the gaps between both rotors or between the rotors and the rotor casing can be reduced even if the rotor surfaces are brought into contact with each other due to complicated thermal expansion during operations, mechanical processing errors, and the like. Therefore, the coating has lubricity, heat resistance, and rust prevention (refer to Japanese Patent Nos. 3267814 and 3740178).
Differences in temperature and pressure between the suction side and the discharge side of the rotors become large in the oil-free screw compressor because there is no medium for cooling friction heat unlike the oil-cooling screw compressor.
The air sucked at substantially at room temperature is compressed to 800 kPa by rotation of the screw. The temperature of the compressed air reaches as low as 260° C. and as high as 360° C. when being discharged by adiabatic compression. Thus, high heat-resistance is required for the coatings applied to the rotor surfaces exposed to the high-temperature air. The coatings are degraded by heat and are separated by contact and sliding of the rotors. Alternatively, the coatings are gradually degraded, separated, and dropped by being exposed to high temperatures for a long period of time.
As described above, if the coatings are separated, the gaps between the both rotors or between rotors and the rotor casing are widened, and the air leaks from the gaps, resulting in deterioration in performance. The leaked air is compressed by rotation of the screw, and the temperature of the air further rises. As described above, if the air leaks, the performance is deteriorated, and the discharge temperature further rises, resulting in a vicious circle.
Further, when the operation of the compressor is stopped, the high-temperature compressed air is cooled to generate dew condensation by condensation of moisture in the air, and moisture possibly adheres to the inside of the compressor. In this case, if the coatings are separated and a base metal portion is exposed, there is a high possibility that the portion tarnishes due to the dew condensation. The rust generated when the operation is stopped causes scuffing at the time of actuating the compressor for the next time and failures of the compressor.
Further, demand for maintenance-free has recently been high for the oil-free screw compressor, and thus development of high-performance and long-life coatings has been required. Therefore, it has been necessary to prevent deterioration in performance of the oil-free screw compressor and scuffing caused by rust by improving the heat resistance of the coatings that is intimately related to degradation and separation of the coatings.
An object of the present invention is to provide a screw compressor including screw rotors with coatings having high solid lubricity and heat resistance.